Achieving human comfort by microenvironmental means is not a new concept. Man has been doing it for eons every time he wore a bearskin in his cave to protect himself against the cold or a coat against the chill of a winter's night in more modern times. These achieve warmth for the wearer because they prevent cool airflow from the outside reaching the skin of the user. Thus the trapped air is warmed by the body heat to near body temperature and the person feels comfortable. However, these garments must also ‘breathe’ and allow a small amount of escape of that warmed air in order to allow the skin's perspiration and its humidity to also escape. Otherwise that moisture is trapped against the skin and keeps the pores of the skin from allowing their moisture to escape effectively. This means our primary mechanism for regulating body temperature (sweating) is inhibited and the person will shortly become uncomfortable.
While microenvironmental heating is something man has done for millions of years, microenvironmental cooling is new. Do you really care what temperature exists in the back corner of the room on a hot summer's day? No, you only care about the 1-inch of air immediately surrounding you skin. You care not only about its temperature, but also about its humidity.
Heat Index is the ‘feels like’ temperature in hot conditions. It's primary components are ambient temperature and humidity. A 90° F. temperature with 40% humidity has a Heat Index “feels like” temperature of 91° F. Raise that humidity level to 90% and the Heat Index rises to 121.9° F. Raise both temperature and humidity to 100° F. and 100% humidity and the Heat Index is 195.3° F. Thus humidity, more that heat itself, is the primary driver of increased Heat Index and heat discomfort.
Whether the air is moving or still is not considered to be a component of Heat Index. Yet anyone working in hot humid conditions knows hot conditions in still air are far more miserable than the same conditions with a 2-3 mph breeze.
The human body's primary means for internal temperature regulation is sweating (transpiration) where body water is put out through the skin's pores as water or water vapor and so rids the body of its internal excess heat. As humidity rises, the gradient between the skin's pores and the outside environment is reduced finally to the point where water vapor can no longer flow from the skin to the outside (at 100% relative humidity). Now the body is inefficient at ridding itself of its heat, and heat begins to build up, causing more sweating as formation of liquid on the skin in an effort to rid the body of its excess heat.
If that person is in still air and the same relative position for long periods of time (as walking will cause the air and its heat/humidity in contact with the skin to move off the skin), his immediate environment becomes saturated with his own body moisture, either as liquid or humid vapor at his body temperature, Regardless of the actual temperature, the 1-inch of air immediately around him becomes nearly 100% humid and at (or even above) his core body temperature. If even a slight 2-3 mph breeze then begins to blow upon him, that humid vapor is blown away from him and he immediately begins to feel relief. His ability to transpire effectively is restored and he begins to feel cooler immediately with both his immediately surrounding temperature and humidity dropping as his personal Heat Index plummets. Comfort is restored.
Air-conditioning of indoor air began in the early 20th Century and is a comfort familiar to us all. Yet certain jobs and outdoor activities leave us uncomfortable as air-conditioning is not possible. Outdoor jobs, work in warehouses where air-conditioning is impractical, work in confining work clothes where one cannot feel air-conditioning such as a gowned surgeon at surgery, workers in HazMat or BioHazard total enclosure suits, firemen, furnace workers, flight-line workers, etc. all share hot sweaty work conditions as the working conditions prevent them from experiencing proper air-cooling. That is—until now.